A vehicle such as an automobile, in order to immediately notify a driver of the occurrence of various unexpected events (for example, a door is left open, a light is left on, etc.) and draw attention of the driver, generates a beep sound by sounding a buzzer. The beep sound has been generated by inputting a square wave at a frequency corresponding to a frequency of a desired sound (for example, 2 kHz) to a buzzer driving circuit and applying the square wave to the buzzer.
In general, such a buzzer driving circuit is configured to receive an input of the square wave and, upon reception of the input of the square wave, causes a large inrush current flowing thereto, which may have an impact to generate a momentarily abnormal noise. Also, at the moment when the input of the square wave is finished, a counter electromotive force generated in the buzzer serving as an inductive load may have an impact to cause the momentarily abnormal noise.
As such, in order to prevent noise contamination by subduing a rapid change in a level of the signal input to the buzzer, a low-pass filter (for example, a CR filter that consists of a resistor and a capacitor) is inserted into an input stage of the rectangular wave, thereby dulling the square wave and preventing the rapid change in the level of the signal input to the buzzer (for example, PLT 1).